Pump

ABSTRACT

A pump includes a power screw and at least one idler screw which meshes with the power screw, the power screw and idler screw being rotatable in a housing, the idler screw having at least one screw form including a generally helical groove and a generally helical land surface, the land surface having a first, and a second edge portion, each of which is adjacent to the or a groove, the distance between the land surface and a longitudinal axis of the idler screw varying between the first edge portion and the second edge portion, the distance between the first edge portion and the longitudinal axis of the idler screw being substantially constant over the length of the screw form and the distance between the second edge portion and the longitudinal axis of the idler screw being substantially constant over the length of the screw form.

[0001] This application claims priority to United Kingdom PatentApplication No. 0226529.6 filed Nov. 14, 2002, the entire disclosure ofwhich is incorporated herein by reference.

BACKGROUND TO THE INVENTION

[0002] The present invention relates to a pump particularly to a pumpincluding a power screw and at least one idler screw mounted in ahousing, the idler screw which meshes with the power screw such thatrotation of the power screw causes rotation of the idler screw.

[0003] Such a pump is commonly known as a screw pump, and pumping offluid within the housing is effected by fluid becoming trapped andpressurised between the meshing screws.

DESCRIPTION OF THE PRIOR ART

[0004] Typically, such pumps include one or two cylindrical idlerscrews. Where two idler screws are provided they are typically mountedon diametrically opposite sides of the power screw. The idler screwsneed not be attached to the housing but may simply be retained withinthe housing. Alternatively, the idler screws may be mounted at an end ina bearing provided in the housing. In both cases, the pressure from thepumped fluid exerts a force on the idler screws which tends to push theidler screws away from the power screw into the housing. As a result,frictional forces between the idler screws and the housing can causesignificant energy losses. This is a particular problem when the idlerscrews are not mounted in a bearing.

SUMMARY OF THE INVENTION

[0005] According to the invention a pump including a power screw and atleast one idler screw which meshes with the power screw, the power screwand idler screw being rotatable in a housing, the idler screw having atleast one screw form including a generally helical groove and agenerally helical land surface, the land surface having a first and asecond edge portion, each of which is adjacent to the or a groove, thedistance between the land surface and a longitudinal axis of the idlerscrew varying between the first edge portion and the second edgeportion, the distance between the first edge portion and thelongitudinal axis of the idler screw being substantially constant overthe length of the screw form and the distance between the second edgeportion and the longitudinal axis of the idler screw being substantiallyconstant over the length of the screw form.

[0006] By virtue of the shape of the idler screw, the pumped fluid actsas a hydrodynamic bearing, supporting the idler screw and reducing thefrictional forces between the idler screw and the housing.

[0007] In one embodiment, the distance between the or the at least oneland surface and the longitudinal axis increases continuously from thefirst edge portion to the second edge portion. In a preferredembodiment, the distance between the or the at least one land surfaceand the longitudinal axis, initially increases from the first edgeportion to a position intermediate the first and second edge portions,and then remains generally constant from the intermediate position tothe second edge portion. The intermediate position may be approximatelyhalf way between the first and second portions. Such an arrangement maybe easier to manufacture and gauge than where the distance increasescontinuously. Also in use, the hydrodynamic pressure drop between wherethe second edge portion separates from the adjacent housing wall as theidler screw rotates, and where the next first edge portion engages thehousing wall is reduced compared within a continuously rampedarrangement.

[0008] In each case, preferably the idler screw is arranged so that thefirst edge portion leads the second edge portion as the idler screwrotates in use.

[0009] The first and/or second edge portion may be slightly radiussed ifdesired so as to provide a lead-in to the or an adjacent groove.

[0010] Preferably the power screw includes at least one generallyhelical ridge which engages with the generally helical groove of the oreach idler screw.

[0011] Preferably, each idler screw includes two generally helicalgrooves of substantially the same pitch and two generally helical landsurfaces, each land surface having a first and a second edge portion,each of which is adjacent to a groove, the distance between each landsurface and a longitudinal axis of the idler screw varying between thefirst edge portion and the second edge portion over at least part of thelength of the idler screw. In this case, the power screw includes twogenerally helical ridges of substantially the same pitch.

[0012] Preferably the pump includes two idler screws located atdiametrically opposite sides of the power screw.

[0013] The difference in the distance between the longitudinal axis ofthe idler screw and the land surface at the first edge portion and thesecond edge portion may be up to 4% of the largest distance between thelongitudinal axis of the idler screw and the land surface.

[0014] An end of the or each idler screw may be mounted in a bearingprovided in the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, of which,

[0016]FIG. 1 is an illustration of a power screw and idler screws and ahousing of a pump according to the invention,

[0017]FIG. 2 is an illustration of a transverse cross-section throughthe power screw and idler screws of FIG. 1.

[0018]FIG. 2a is a view similar to FIG. 2 but showing a preferred idlerscrew form;

[0019]FIG. 3 is an illustration of a longitudinal section through a pumpaccording to the invention, in the direction shown by arrow X in FIG. 2.

[0020]FIG. 4 is an illustration of a longitudinal section through a pumpaccording to the invention in the direction shown by arrow Y in FIG. 2.

DESCRIPTION OF THE PROPOSED EMBODIMENTS

[0021] Referring now to the figures, there is shown a power screw 10 andidler screws 12 of a screw pump, the two idler screws 12 located atdiametrically opposite sides of the power screw 10. Longitudinal axes Aof the idler screws 12 are generally parallel to the longitudinal axis Bof the power screw 10. Each idler screw 12 has two screw forms whichinclude first and second generally helical grooves 14, 14′ which eachextend along substantially the entire length of the screw 12. The twogrooves 14, 14′ are both of substantially the same pitch and sense, andthe turns of the grooves 14, 14′ are interposed such that when the screw12 is viewed in transverse cross-section one groove 14 is diametricallyopposite the other 14′.

[0022] The grooves 14, 14′ are separated by first and second generallyhelical land surfaces 16, 16′, each land surface having a first edgeportion 16 a, 16 a′ and second edge portion 16 b, 16 b′ adjacent to oneof the grooves 14, 14′. The first edge portion 16 a of the first landsurface 16 is adjacent to the second groove 14′ and the second edgeportion 16 b of the first land surface 16 is adjacent to the firstgroove 14. The first edge portion 16 a′ of the second land surface 16′is adjacent to the first groove 14, and the second edge portion 16 b′ ofthe second land surface 16′ is adjacent to the second groove 14′.

[0023] In the FIG. 2 embodiment, the distance between each land surface16, 16′ and the longitudinal axis A of the idler screw 12 increasescontinuously from the first edge portion 16 a, 16 a′ to the second edgeportion 16 b, 16 b′.

[0024] In the embodiment of FIG. 2a, on which all like parts to those onFIG. 2 are labelled with the same references, the distance between eachland surface 16, 16′ and the longitudinal axis A of the idler screw 12increases from the first edge portion 16, 16 a′ continuously to anintermediate position I, I′, which intermediate position I, I′ is inthis example, approximately half way between the respective first andsecond edge portions 16 a, 16 a′; 16 b, 16 b′. From the intermediatepositions I, I′ to the second edge portions 16 b, 16 b′, the distancebetween the land surface 16, 16′ and the longitudinal axis A of theidler screw 12 is generally constant.

[0025] In each example, if desired, the first and/or second edgeportions 16 a, 16 a′; 16 b, 16 b′ may be slightly radiussed so as toprovide a lead-in to the adjacent respective groove 14, 14′.

[0026] Typically, the difference in the distance between the first edgeportion 16 a, 16 a′ of each land surface 16, 16′ and the longitudinalaxis A and the distance between the second edge portion 16 b, 16 b′ ofeach land surface 16, 16′ and the longitudinal axis A is up to 4% of thelargest outside diameter of the idler screw 12, i.e. the distancebetween the second edge portion 16 b, 16 b′ of each land surface 16, 16′and the longitudinal axis A. Thus it is apparent that this differencehas been exaggerated in FIGS. 2 and 2a for clarity.

[0027] The power screw 10 and idler screw 12 are both generallycylindrical, however, and thus it will be appreciated that the distancebetween each first edge portion 16 a, 16 a′ and the longitudinal axis Aof the idler screw 12 is substantially constant over the length of theidler screw 12, as is the distance between each second edge portion 16b, 16 b′ and the longitudinal axis A.

[0028] The power screw 10 is provided with first 18 and second 18′generally helical ridges which each extend along substantially theentire length of the screw 10. The two ridges 18, 18′ are both ofsubstantially the same pitch & sense, and the turns of the ridges 18,18′ are interposed such that when the screw 10 is viewed in transversecross-section, one ridge 18 is diametrically opposite the other 18′.

[0029] The first ridge 18 meshes with the first groove 14 of each idlerscrew 12, and the second ridge 18′ meshes with the second groove 14′ ofeach idler screw 12, and thus rotation of the power screw 10 about itslongitudinal axis B in the direction indicated by the respective arrow,causes rotation of the idler screws 12 about their longitudinal axes Ain the opposite rotational direction as indicated by the respectivearrows.

[0030] The power screw 10 and idler screws 12 are mounted in a housing20, part of which is illustrated in FIGS. 2 and 2a, 3 & 4, which isshaped to provide minimum clearance between the screws 10, 12 and thehousing 20, whilst permitting rotation of the screws 10, 12 within thehousing 20. The housing 20 thus includes an elongate cavity in which thescrews 10, 12 are housed. In cross-section, the cavity includes acentral part circular portion 20 a of a larger diameter adapted to housethe power screw 10, and two part circular lobe portions 20 b located ondiametrically opposite sides of the central portion of 20 a, which eachhave a smaller diameter and are adapted to house the idler screws 12.

[0031] As illustrated in FIGS. 3 & 4, the housing 20 also includes anoutlet port 22 and an inlet port 24, the inlet port 24 being arrangedsuch that fluid enters the cavity adjacent to first ends of the drivenand idler screws 10, 12 and the outlet port 22 being arranged such thatfluid leaves the cavity adjacent to second ends of the driven and idlerscrews 10, 12. The inlet port 24 is connected to a supply of fluid to bepumped.

[0032] The power screw 10 is connected to a motor (not shown) by meansof a drive shaft 26, the drive shaft 26 being mounted in bearings 28provided in the housing 20. Operation of the motor causes the powerscrew 10 to rotate about its longitudinal axis B in a first sense, andwhich thus causes the idler screws 12 each to rotate about theirlongitudinal axes A in an opposite sense to the power screw 10. Thescrews 10, 12 are oriented such their rotation causes fluid to be drawnfrom the inlet port and moved along the length of the screws 10, 12 tothe outlet port. Fluid trapped between the ridges 18, 18′ and thegrooves 14, 14′ as the screws 10, 12 mesh becomes pressurised and exertsan outward force on both the idler screws 12, which tends to push theidler screws 12 towards the housing 20.

[0033] The idler screws 12 are configured such that the first edgeportions 16 a, 16 a′ of each land surface 16, 16′ lead the second edgeportions 16 b, 16 b′ as the idler screws 12 rotate. The resulting changein separation between the land surfaces 16, 16′ and the housing 20compresses the fluid between the housing 20 and the land surfaces 16,16′, the resultant local fluid pressures enable the fluid to act as ahydrodynamic bearing, supporting the idler screws 12, and reducingfrictional forces between the housing and the land surfaces 16, 16′ ofthe idler screws 12. Thus, the efficiency of the pump is improved.

[0034] Such a pump advantageously is used to pump fluid for hydraulicsystems which may include actuators.

[0035] In the examples described, the idler screws 12 are not connectedto the housing 20, but it is possible to mount the idler screws 12 onbearings connected to the housing 20.

[0036] It is not necessary to provide each idler screw 12 with twogrooves 14, 14′ and the power screw 10 with two ridges 18, 18′. Eachidler screw 12 may be provided with one or more than two grooves 14,14′, and the power screw 10 provided with a corresponding number ofridges 18, 18′.

1. A pump including a power screw and at least one idler screw whichmeshes with the power screw, the power screw and idler screw beingrotatable in a housing, the idler screw having at least one screw formincluding a generally helical groove and a generally helical landsurface, the land surface having a first and a second edge portion, eachof which is adjacent to the or a groove, the distance between the landsurface and a longitudinal axis of the idler screw varying between thefirst edge portion and the second edge portion, the distance between thefirst edge portion and the longitudinal axis of the idler screw beingsubstantially constant over the length of the screw form and thedistance between the second edge portion and the longitudinal axis ofthe idler screw being substantially constant over the length of thescrew form.
 2. A pump according to claim 1 wherein the distance betweenthe or the at least one land surface and a longitudinal axis increasescontinuously from the first edge portion to the second edge portion. 3.A pump according to claim 1 wherein the distance between the or at leastone land surface and the longitudinal axis initially increases from thefirst edge portion to a position intermediate the first and second edgeportions, and then remains generally constant from the intermediateposition to the second edge portion.
 4. A pump according to claim 3wherein the intermediate position is approximately half-way between thefirst and second edge portions.
 5. A pump according to claim 1 whereinthe idler screw is arranged so that the first edge portion leads thesecond edge portion as the idler screw rotates in use.
 6. A pumpaccording to claim 1 wherein the first and/or the second edge portionsinclude a radiussed lead-in to the or an adjacent groove.
 7. A pumpaccording to claim 1 wherein the power screw includes at least onegenerally helical ridge which engages with the generally helical grooveof the or each idler screw.
 8. A pump according to claim 1 wherein eachidler screw includes two generally helical grooves of substantially thesame pitch and two generally helical land surfaces, each land surfacehaving a first and a second edge portion, each of which is adjacent to agroove, the distance between each land surface and a longitudinal axisof the idler screw varying between the first edge portion and the secondedge portion over at least part of the length of the idler screw.
 9. Apump according to claim 8 wherein the power screw includes two generallyhelical ridges of substantially the same pitch.
 10. A pump according toclaim 1 wherein the pump includes two idler screws located atdiametrically opposite sides of the power screw.
 11. A pump according toclaim 1 wherein the difference in the distance between the longitudinalaxis of the idler screw and the land surface at the first edge portionand the second edge portion is up to 4% of the largest distance betweenthe longitudinal axis of the idler screw and the land surface.
 12. Apump according to claim 1 wherein an end of the or each idler screw ismounted in a bearing provided in the housing.